With currently available chemotherapeutic agents most patients with untreated A.M.L. can achieve an initial complete remission. The majority of these patients, however, eventually relapse and are usually refractory to treatment with the same agents. In general, the degree of response to antineoplastic is associated with the ability of a leukemic cell to accumulate and retain the drug. Hence, studies directed to define abnormalities in membrane transport, can improve an understanding regarding drug-cell interaction as well as drug cytotoxicity and selectivity. Our current aims are to take advantage of the D.V.M. capabilities to examine a heterogeneous population of cells, and determine in vitro as well as in vivo uptake and retention of Daunomycin in individual leukemic cells. Further, an attempt will be made to correlate these data, with drug cytotoxicity, and plasma and cell drug levels as measured by a stem cell assay and HPLC respectively. In addition, we will study selected patients with solid tumors receiving Adriamycin, to determine whether drug levels measured in peripheral and bone marrow cells can be used as predictors of drug toxicity.